The following U.S. Pat. Nos. exemplify the state of the art of the safety control valves:
______________________________________ 2,407,761 3,010,691 2,480,712 3,138,172 2,691,504 3,459,403 ______________________________________
In these prior art devices the general concept is the use of a diaphragm or piston of various designs selectively interposed in the fluid flow for control purposes. The piston or diaphragm is operated either by a pressurized fluid or by a portion of the pressurized fluid being controlled by the valve itself. These devices suffer the common drawback of uncertainty in the supply of fluid under pressure which is required to operate the valve. Also, these prior art devices tend to act too slowly to protect a high vacuum from reentry of air or contamination by oil from the vacuum pump.